1. Field of the Invention
This invention relates to the processing of periodic information signals, such as, video signals, and more particularly is directed to apparatus by which time base errors introduced during recording and/or reproducing of such signals may be removed.
2. Description of the Prior Art
Video signals are frequently recorded on magnetic tape and subsequently reproduced for later broadcasting or viewing purposes. During the reproduction of recorded video signals, time base or frequency errors are usually introduced by reason of expansion or contraction of the record medium during or after recording, variation in the speed of the tape relative to the magnetic head or heads during recording or reproduction, variation between the tape recording speed and the tape reproducing speed, and the like. The presence of such time base errors in the reproduced video signals cause a frequency shift of the latter which can result in many observable undesirable effects, particularly when the reproduced video signals are to be transmitted or broadcast and may be mixed with live broadcast material that do not have such time base errors. The observable undesirable effects resulting from relatively small time base errors are a smeared or jittery picture with erroneous intensity variations and, in the case of color video signals, improper color display. When the time base errors are large, the reproduced picture will fail to lock horizontally or vertically.
Time base correctors are known for the purpose of substantially removing time base errors from video signals. These time base correctors usually employ tapped delay lines, or other variable delay elements, for introducing variable delay in the incoming video signals in order to compensate for the undesired frequency variations or time base errors in such incoming signals. Such time base correctors employing variable delay elements are only capable of correcting relatively small time base errors of the magnitude that are encountered when recording and/or reproducing the video signals by means of an apparatus having four rotary heads. However, when the video signals are recorded and/or reproduced by means of an apparatus having only two alternately operative rotary magnetic heads, the possible magnitudes of the time base errors in the reproduced video signals are too large to be corrected or compensated for by means of the variable delay elements.
In view of the foregoing, it has been proposed, for example, as disclosed in U.S. Pat. No. 3,860,952, issued Jan. 14, 1975, to provide a time base corrector for processing video signals in which the incoming video signals are converted from analog to digital form and temporarily stored in a memory unit. Time base errors are removed from the video signals by writing the digitized signals in the memory unit at a clocking rate which varies in a manner generally proportional to the time base errors, and by fetching or reading out these stored signals at a standard clocking rate. After such reading out of the digitized video signals, the latter are reconverted to analog form and applied to an output terminal. The memory unit used as aforesaid comprises a plurality of memories or shift registers each capable of storing a plurality of horizontal lines of video information, and a sequence control unit controls the selection of each memory for writing and reading in such a manner that double clocking of a single memory which marginally occurs at the extreme boundaries of the correction range is relieved. However, the arrangement disclosed in the above identified patent for preventing double clocking of a single memory, that is, an attempt to read and write contemporaneously from the same memory in response to an excessive time base error, results in at least one incomplete or otherwise deteriorated line interval signal, and possibly even two incomplete or deteriorated line interval signals which are out of horizontal synchronization with each other and which are present in the output from the time base corrector. Further, the known time base corrector is not capable of eliminating from its output those line intervals of the incoming video signals in which drop outs may occur.